Radar transponders, repeaters, and the like are constructed to transmit a reply in response to a received signal but should not reply to interference, extraneous signals, man-made interference and the like. In general, prior art structures do not distinguish between input frequencies but simply disable the receiver upon the reception of a strong side lobe signal. Thus, a strong side lobe signal at one frequency will disable the receiver for weak main lobe signals at a different frequency, even when the weak signal frequency is within the receiver input bandwidth.